Add servletcontext as an argument to responseAsJson
Add a proper test to make sure that the proposed change is necessary and works as advertised

1. Ensure that the synchronous version works
2. Ensure that the asynchronous version using AsyncResult works
Replace servletContext with scalatraContext as variable name because it is more appropriate
Merge pull request #127 from lloydmeta/feature/jsonFeature_futureable

Add servletcontext as an argument to responseAsJson